Coil coatings are applied to coiled sheet metal stock, such as steel or aluminum, in an economical, high speed process. The coil coating process results in a high quality, uniform coating with little waste of the coating and little generation of organic emissions as compared to other coating methods, e.g. spray application of a coating composition.
Coil coating is a continuous feeding operation, with the end of one coil typically being joined (e.g., stapled) to the beginning of a next coil. The coil is first fed into an accumulator tower and after coating is fed into an exit accumulator tower, with the accumulator towers allowing the coating operation to continue at constant speed even when intake of the steel is delayed, for example to start a new roll, or winding of the steel after coating is delayed, for example to cut the steel to end one roll and begin a new roll. The coil is generally cleaned to remove oil or debris, pre-treated, primed with a primer on both sides, baked to cure the primer, quenched to cool the metal, then coated on at least one side with a topcoat. A separate backer or a different topcoat may be applied on the other side. The topcoat is baked and quenched, then fed into the exit accumulator tower and from there is re-rolled.
One of the controlling factors for the coil coating line speed is the oven dwell time necessary to cure the applied coating at the cure oven temperature. A coating composition that can be cured in a shorter time at the oven temperature allows a faster and more economical coil coating process. A number of other properties are important for coil coatings, too, such as resistance to degradation on outdoor exposure (weatherability), chemical resistance, water resistance, scratch resistance, gloss, hardness, and resistance to delamination when the substrate is bent. The bending property is important because after being coated the metal is subjected to a forming step. For example, building panels are formed into a three-dimensional shape after coating. It is important that the coating not lose adhesion during the forming step or steps. Weatherability is important for metal that will be used for building panels, gutters, garage doors, sign stock, panels used for vehicle parts, or other such uses where the coated surface is exposed to outdoor weather and sun. While the bending property is generally better with softer, more flexible binders, weatherability and other durability properties are generally better with harder binders.
Various coil coating compositions have been proposed to be suitable for the coil coatings process and provide the different coating properties desired. For example, Heyenk et al., U.S. Pat. No. 6,413,648 B1 describes a thermosetting coating composition containing a mixture of two polymers, one of which is amorphous with a glass transition temperature greater than about 45° C. The polymers may be linear or branched polyacrylates or polyesters. Tachika et al., U.S. Pat. No. 5,563,223 describes a composition that balances the need for processability of the coating composition with the need for alkali resistance, gasket resistance, weatherability and resistance to staining in the cured coating. The composition contains a curing agent and a polyester prepared using an acid component that is at least 50 mole % aromatic dicarboxylic acid and a glycol component having 1–25 mole % 2-methyl-1,3-propanediol and 75–99 mole % alkylene glycol having 5 to 10 carbon atoms. Alternatively, the glycol component can be 20–85 mole % of alicyclic glycol, 80–15 mole % of the addition product of bisphenol A and alkylene oxide, and up to 50 mole % of other glycol(s). Sullivan, U.S. Pat. No. 5,380,816 discloses thermoset coating compositions containing linear polyesters consisting essentially of recurring units of isophthalic acid, an aliphatic diol component including 2-methyl-1,3-propanediol, and, optionally a further dicarboxylic acid. The cured coatings reportedly have improved flexibility and hardness, although it requires a cure time that is comparatively long for a coil coating. Toman et al., U.S. Pat. No. 4,968,775 discloses a thermosetting coil coating composition containing an aminoplast resin and a polyester prepared by condensation of 2-methyl-1,3-propanediol, neopentyl glycol, isophthalic acid, and terephthalic acid, and may contain 1,6-hexanediol or other symmetrical glycol, trimethylolpropane, adipic acid or other symmetrical aliphatic dicarboxylic acid, and/or trimellitic anhydride. The Toman composition is reportedly particularly resistant to crystallization. Finally, Yamada et al., U.S. Pat. No. 4,734,467 discloses a coil coating composition consisting essentially of a crosslinking component selected from melamine resin or isocyanate compound and a mixture of linear and branched polyester resins. The cured coating is reported to have desirable hardness, bending, processability, fastness to boiling water, weather resistance, chemical resistance, and marker stain resistance.
While these compositions offer certain advantages, the cure time remains a problem. A coating composition cures at a given peak metal temperature. While the peak metal temperature can be reached more quickly if the oven temperature is high, there are practical and economic limitations on how high the oven temperature can be. The Heyenk patent Example II of a coil coating has a curing cycle of 42 seconds at 358° C. (676° F.) to reach a peak metal temperature of 232° C. (450° F.) and Example VI provides a coil coating cured for 41 seconds at 300° C. (572° F.) to reach a peak metal temperature of 241° C. (466° F.). The Tachika patent example coatings, particularly topcoat, are cured for 60 seconds at 230° C. (446° F.). The Sullivan patent example coatings were reported as baked at 245° C. (473° F.) for 75 seconds or at 240° C. (464° F.) for 125 seconds. The Toman patent examples do not provide a cure schedule or peak metal temperature. The Yamada patent example coatings were reported as baked at 230° C. (446° F.) for 40 seconds. A reduction in these curing times would allow a faster, more economical coil coating process.
It would be advantageous to be able to fully cure the coil coating with a shorter dwell time, while maintaining adequate bending and other properties